Apparatus for determining thermal stability of materials



May 8, 1951 H. R. MOORE 2,551,624

APPARATUS FOR DETERMINTNG THERMAL STABTLITY oF MATERIALS Filed sept. 28,1945 l POWER SUPPLY AAC. 0l? 0.6.

IN VEN TOR. How@ m /Vl Oor@ Patented May 8, 1951 UNITED STATES PATENTOFFICE APPARATUS FOR DETERMINING THERMAL STABILITY OF MATERIALS(Grantedunder the act of March 3, 1883, as amended April 30, 1928; 370O. G. 757) 8 Claims.

This invention relates to an apparatus for measuring the thermalstability of materials, and more particularly the minimum temperaturesrequired to effect permanent changes in the physical properties oforganic and inorganic materials in the solid state.

The apparatus is used to make determinations of the threshold burningtemperatures of organic compounds and resinous polymers of the mostdiversified character. It is useful in determining the melting points ofinorganic compounds, such as oxides and silicates. The thermal stabilityof synthetic inorganic pigments similar to ultramarine blue can beinvestigated with its aid.

An object of the invention is to provide a plurality of metal supportsof different resistivities and designs to serve as substrata for thesolid materials to be tested.

Another object is to provide electrical power sources capable offurnishing controlled and unform sources of heat to metal supports ofdifferent resistivity and design so that the materials tested are inimmediate contact with flat metal surfaces having temperatures of 100 to3300 C.

Another object is to provide a finished apparatus comprising partsessential to effective utilization of the principle of transformingelectrical energy into heat energy for measuring accurately and in areproducible fashion the reresistant properties of organic raw materialsand nished products of the most diversied character.

The foregoing objectives are realized with the assembly of parts andcombinations hereinafter set forth and claimed.

The principles of operation and type of data obtained with this deviceare most conveniently illustrated by its application to determinationsof the burning tendencies of organic materials. Visible burning occurswhen the rate of combustion of materials attached to the metal supportsis sufficiently rapid to give visible flames at normal pressures andtemperatures.

An immediate use of the apparatus is to provide quantitative data on theburning properties of the following classes of finished productssubjected to a controlled source of heat for definite periods of time:surface coatings, plastic sheeting, linoleum, cements, adhesives,electrical insulaticn, silicone resins, phonographic discs, decorativematerials (chemically treated), film base (non-flammable), woodpreservatives, Water-proofing materials, textile preservative compounds,floor waxes, gasket materials, sheet materials fabricated from neopreneand other synthetic rubbers.

Heretofore, in the art of measuring the fire retardance of solid organicmaterials, it has been customary to employ gas burners or blast lampsfor the direct application of heat to the specimens. Electric mulefurnaces have also been used, in which the backs of sheet-metal supportsfor test material have been substituted for the furnace doors. None ofthese methods is satisfactory, since they fail to furnish a source ofheat that is sufficiently uniform, rapid and intense to meet the severeservice conditions required of nre-resistant materials.

Temperatures in excess of 1800 F. rarely can be obtained by methodshitherto used. Furthermore, older methods are inherently deficient inproviding a source of heat that is constant in a series of tests, sothat it is impossible to secure reproducible results.

As a consequence of the foregoing defects, the use of older methods doesnot insure the heating of solid materials to suiciently hightemperatures, with sufcient speed, to prevent the loss of volatilecombustible ingredients by vaporization or by gaseous diffusion out ofthe organic matrix. Owing to these basic effects, the older methods givemisleading results and higher ratings for fire retardance than thevarious compositions are entitled to receive. A unique feature of thenew method is its capacity to supply heat of the requisite intensity,very rapidly, to ignite combustible ingredients in the materials tested.

The older sources of heat derived from sources external to the supportsfor the test specimen are all inferior to the method developed in myinvention which depends on the generation of heat within a metallicsupport and the test material attached thereto by the preferred methodof utilizing the heating effect of an electric current in metal stripsupports of special design. High frequency induction can also be used tosecure controlled rates of heating and temperatures short of fusion butthis method is not as convenient or cheap as the preferred method.

The basic principle of my invention is an application of the principleof the transformation of electrical energy into heat energy to provide auniform source of heat, applied at a controlled rate, to individualmetallic substrata for the various materials tested. Flammabilitycharacteristics are then defined in terms of the minimum quantity ofelectric current that ignites specimens. afIiXed to the test strip andproduces the first detectable evidence of luminous flames.Alternatively, flammability is defined in terms of the minimum lengthsof time required to ignite test specimens in direct contact withdifferent metal substrata heated by constant currents.

rfhe validity of the application of the fundan mental equation for thetransformation of electrical into heat energy to this invention isconfirmed in a semi-quantitative manner by com.-

paring the calculated temperature increases of the metallic stripsubstrata with the observed values determined by means of the opticalpyrometer. These calculations verify the application of the familiarequationI v E2 t H*"TX4.18

to this invention. Estimates of the heating eff ect in calories, H, canbe used to calculate the degrees temperature rise from published data onthe heat capacity of the metal as a function of temperature, the mass,length and cross section of the strip, the specic heat and specificresistivity of the metal under consideration.

'l ,Inorder to make the invention more clearly understood, thereis`shoWn in the diagram, Fig. 1,y means for carrying out the inventioninto practical use. This arrangement of essential parts isV given as a,means of illustration only and does not limit the construction of afinished test unit to this particular construction. Thus, the essentialelements comprising the unit can be varied in proportion and inarrangement without departing from the basic principles and scope of theinvention as defined in the appended claims.

Fig. 2 shows a special adaptation, in perspective, of-l theI testspecimen.

A practical embodiment Vof the thermoelectric principle is also given inNavy Department Specication 27-L-12, under date of March 1, 1945, forbattleship linoleum, re retardant. The invention includes a gas burnerto ignite any combustible gases released by the 1/g-inch-thick linoleumspecimens tested. The gas burner provides a. non-luminous ame by theburning of gas such as commercial propane gas. The gas flame is adjustedto be about one inch from the test specimen, and to extend lengthwiseapproximately parallel to the specimen for a distance of about fiveinches. A suction blower is also provided to remove'smoke and fumesevolved during a run. The foregoing specification gives furtherinformationl onv operating 1 details and thermal stability propertiesascertained with the thermoelectric apparatus, as a means of insuringthe procurement of .a suitable grade of fire-resistant linoleum forships.

fThe separate vitems of equipment and parts of the assembly, asillustrated schematically byFig. 1,v are:A

y(1) An,A..C. or D, C. power supply.

(2l An auxiliary 110 volt, 60 cycle, current ing through the specimen,utilizing a suitable shuntfor the D, C. power source and a currenttransformer for the A. C. power supply.

(8) A, C. and D. C. voltmeters for measuring the voltage drop across thespecimen depending on. the power Supply..

(9) Copper or brass terminal bars with jaws and-- clamps for affixingthe metallic test specimen, one bar to be Xed and the other movable toallow for expansion and contraction of the support.

(10) Ar metallic test support, with or without material subjected totest.

' The steelA support shown, in Fig. 2 is cut from hot or cold rolledstrip stock 13 inches X 3 inches in size With a constriction 6 incheslong by 1 inch wide for application ofv the test material. Linoleum` orother plastic sheet materials are affixed with wires to this' narrowconstriction which attains aA uniform temperature over its entire 6 inchlength because of its higher resistivity.

Peak temperatures of 2600 F. are obtained with steel specimens 0.017 i0.003 inch in thickness, although still higher temperatures of 2900 to3000 F. are obtainedv with specimens ranging from 0.0123r to 0.0139 inchin'thickness.

In practice it has been found that thin sheet stock of steell or ferrousalloy sheets can be used only if provision is made in the design of theterminal bars to accommodate a pulling device to keep the pieceperfectly straight as it expands on heating. This device can take theform of a combination of a stationary terminal bai' and a movable head,The movable head is controlled with a spring or air cylinder whichgradually draws` the piece over as it expands. In this manner, it ispossible to allow for thermal eX- pansion during heating. Anair-cylinder device is preferred since it provides an automatic releaseto permit contraction of the specimens after the current is turned on atthe conclusion (a) Open-circuit voltage 75-80 volts. (b) Welding-Currentrating 600 amperes.

(c) Kilowatts at rated load- 2O kva. (d) Primary voltage r 220 to 440volts. (e) Primary current 91 to 1,82 amperes at rated load. (j) AVmanually operated current regulator.

Alternately, a suitable D. C. power supply com- `port and itsdimensional characteristics.

prises a Welding generator with a current rating of 600 amperes, thecurrent delivery of which is capable of regulation in small incrementsor in a continuous manner.

The specifications for the power source required by this invention are adirect function of the type of material chosen for the metal sup- Thedesign illustrated by Fig. 2 is preferred for the majority of tests,although it is recognized that more compact, inexpensive and readilyoperated power sources can be utilized with supports of reduced lengthand cross sectional area.

In operating the test unit with either power source, the procedure afterinserting a blank metal specimen is as follows:

(l) Adjust the current regulating device, provided with either the A. C.or D. C. power supply, to deliver a moderate amount of currentcorresponding to 150 to 275 smperes. These quantities of current importa fairly uniform temperature to steel specimens 0.017 inch thick for aperiod of at least three minutes.

(2) Commence the test by turning switch C to the on position, therebyactuating contacter A which starts timer T due to closing of contacterB. Contactor B is normally open, but closes due to the voltage drop inthe test circuit.

(3) Observe the ammeter and voltmeter at the instant of throwing switchC in order to obtain the zero time starting current and voltage dropacross the specimen.

(4) Obtain the workability range of the test specimen for differentcurrent regulator settings by recording the current and voltage readingsat to 20 second intervals over a period of 3 to 4 minutes. As pointedout, supra, these data are most conveniently obtained with an automaticgraphic recorder which gives permanent continuous records of the currentand voltage readings at denite time intervals.

(5) Open the test circuit by turning switch C to the on position, anoperation which demagnetizes the coils of contactors A and B and stopstimer T and the ow of current through the test specimen. The sameoperations occur by fusing the specimen.

Current and voltage readings are useful in computing the electricalresistance of the test supports at any time after closing switch C.These data are plotted as reference curves for estimating the amount ofheat supplied experimental materials in repeat tests.

The same procedure is repeated in determining the fusion times of blankspecimens for starting currents in excess of 275 amperes. Fusion timesthus obtained for starting currents in excess of 275 amperes owingthrough test specimens 0.017 inch thick are given by For all practicalpurposes, the voltage drops for starting currents ranging from 150 to275 6i' amperes4 show only very small increases after the current isallowed to flow through the test supports for a period of 90 seconds. Inother words, the resistivity corresponding to these constant currents ispractically unaifected as the strip is continuously heated, and acondition of substantially uniform temperature is set up. On the otherhand, starting currents of 300 to 600 amperes produce constantlyincreasing temperatures as the specimens are heated until fusion occurs.Nevertheless, operation of the device at these higher currents isindispensable for the evaluation of surface coatings and other materialssuch as plastic sheeting whose thickness does not exceed 3*-2 inch,because the sudden acquisition of high temperatures ignites any solidcombustible materials present or volatile decomtained with aplatinum-platinum rhodium thermocouple attached to the support. Otherdevices such as the radiation pyrometers coupled with Micromax recordersmanufactured by the Leeds and Northrup Company can be used, but are notfavored because of time lag and the dependance of readings on thedistance of the pyrometer from the strip and variable areas selected forobservation.

The use of platinum-platinum rhodium thermooouples in conjunction withthe recording photoelectric potentiometer is superior to radiation andoptical pyrometers since it permits simultaneous readings of thetemperature of the metal support beneath the test material. Thisprocedure is followed with organic materials exceeding 0.01 inch inthickness when it is incorrect to assume that the temperature of thestrip is identical with the value given by the timetemperature referencecurve for uncoated, or blanlfzj metal supports. The higher temperaturesusually obtained under these circumstances for the same current, voltageand time readings are due to the thermal insulating properties of thetest material. These temperature differences are proportional to thethickness of the materials tested and inversely proportional totheirthermal conductivity.

Table 2 gives time-temperature reference data for uncoated supports indegrees Fahrenheit and the times in seconds which corresponds to variousstarting or zero time currents for mild or coldrolled steel specimensaveraging 0.017 inch in thickness. These data were obtained with theassistance of an optical pyrometer for different zero time startingcurrents. The results are valuable in illustrating the principles of theinvention, although it is recognized that more accurate data areobtainable with thermocouples and a suitable recording instrument withnegligible time lag for starting currents in excess of 275 amperes, asexplained above. All such improvements in applying the best availableinstrumentation for recording the basic data of temperature, current,voltage and time are considered within the scope of this invention.

acercar Tables? Zero Time Startins:

Temperature, Degrees Fahrenheit, Time, Seconds Currents,

Amperesy 7 l0 12 ero 1, 04o 1, 250

2, oso

2,890* at v 8 scc.

stratum which permits ignition of the thermal decomposition productsvoiY the solid organic binder, inl situL before they caribe driven ofi.by heat, is. en. outstanding feature of. this. invention.

In practice, the majority of determinations are made by the,Aconstant-current technique which insuressteady telnr)eraturesl of 200 to2000 F. for heating periods of 3 minutes or more without fusiony of theIsupport. In,v this method, organic materialsaiiixed to the metal stripare subjected to a substantially constant heat source, and the minimumtime, required for visible burning to.

take place and the duration of burning recorded. This technique affordsa uniform source of heat ofsufilcient intensity for the majority oforganic materials of thicknessV varying from 0.0005 inchV for surfacecoatings` to 0.125 inch for battleship linoleum.

The results obtained with blank specimensV further show that the testunit also includesv provision-s for-determining i'lreretardance as afunction of the firstl visible detection of burningI when startinglrcurrents of- 300vv to 600 amperes are impressed on the test specimens.In procuring data by this procedure, the zero time currents aregradually.- increased from 300 amperes tothe criticalor thresholdcurrent minimum that just,

permits ignition oithe experimental, material. These critical currentVvalues and the duration ofburningafter the current is,V turnedQcornprise the criteria of flammabilityby thismethod.

The foregoing technique for determining threshold currents is intendedlfor surface; coat.- ings and thi-n sheets of; plastics andother organicmaterials, designed to meet theA most severe requirements, for fireresistance.4 The upper limit.v

of thickness of` driedl organic suriaceA coatings, or other organicmaterials.. ofthe most refractory character, as eXempli-ed, by thesilicone,v

resins,l evaluated by thisy procedure isin the neighbQIThOQdof 0.01,inch. Fire-retardant materials exceeding this thickness can be moreconveniently evaluated by the constant4 current technique.v

The threshold current techniqueis also used to determine the varyingdegrees of solvent retention by surface coatings. The procedureistomeasureV displacements of the minimum currents required for burning tohigher values as the thinner is gradually eliminated by longer periodsof air drying or. by baking. Several tests must be made` of the samecoating appliedV to different strips and allowed toI air dry or bake forvarious times in order to secure signicant. results.

It will be noted that the results obtained by both techniques, constantand threshold cur.- rents, are frequently expressed in terms of thestartingor zero time Values obtained at the instantoi closing switch C,Fig. 1. But the use oi an automatic, graphic recorder of time, currentand voltageas described, supra, ispreferable since the instrument chartsgive the ltruecurrents flowing at the time burning is. ii'rst noticed.Expression` of flammability in terms of the actual current iowing at thecritical instant of rst burning. constitutes a more accurate criterionof comparativel nre-retardant characteristics than thev variousv Zerotime current settings obtained. with the current regulator controlfurnished withV tion for the high-resistance heated area, illus-Ytratedy by Fig. 2, is essential. Investigations-with powdered materialsrequire that the longitudinal edges be upturned one eighth of an inch toretain these materials when the support is placed in ahorizontal'position.

The geometry of the test strip should be apportioned toi the maximumload or duty cycle of the available electrical power supply.

Thus a power supply with a maximum current rating of only 200`amperessuinces to heat a steel test strip 0.012 inch thick if the width andlength of the constricted area is reduced to 0.75 and 4.00 inches,respectively. This strip has only one third the volume of the 6 inchlong X 1 inch Wide, 0.017 inch thick support used to procure thesdata ofTable 2. A strip havingthese dimensionsfor the constricted'portionrequires a much less erg--v pensive power source, although theobservation area is appreciably less.

Other variations in the design of the test strip are useful in obtainingspecial information on the ignition and burning characteristics oforganic materials. For example, horizontal or vertical ns originatingfrom the constricted area and covered with the test material are usefulin measuring the amount of horizontal or vertical propagation of flameafter ignition occurs and the current is turned oir.

An essential qualification of the type of sheet metal chosen forfabrication of the test specimen supports of this invention is that itpossess high temperature coeiiicients of resistivity, so thatsufficiently high temperatures can be readily obtained for evaluatingthe most refractory materials without taxing the current source. Asecond requirement is that it be inexpensive since thermal expansion andsurface oxidation reactions usually render the same specimens unsuitablefor more than one test run. Annealed hot or cold-rolled steel specimensare favored for this purpose, because their resistivity increasesninefold to tenfold in the range of 2500 to 2600 degrees Fahrenheit.

While steel specimens are preferred because they can be economicallydiscarded after each test run, nichrome sheet specimens also can be usedfor repeat runs at temperatures below 1800 F. where expansion andsurface chemical effects are less objectionable. Nichrome and otherhighresistance alloys have the outstanding advantage of permitting alarge number of repeat runs with the same specimen below certaincritical temperatures when allowance is made for expansion effects. Theoccurrence of surface oxidation reactions resulting in the formation ofre scale and the accompanying changes in resistivity prevent the use ofsteel for repeat runs for temperatures exceeding 900 F.

Platinum supports can be used advantageously for a large number oftests, particularly for inorganic compounds and pigments. The samespecimen can be used repeatedly in this case. However, the use ofplatinum is not recommended for evaluating organic materials sincecarbonaceous gases, such as carbon monoxide resulting from theincomplete combustion, have a deleterious eiect on the metal.

The effect of exceptionally high temperatures, in the range of 2000 to3000 degrees centigrade, on various oxides, silicates and otherinorganic compounds can be investigated with the aid of metal supportsconsisting of sheet molybdenum, tantalum of tungsten. Tungsten sheetstock is only presently available as 1/4 inch ribbon, however. Testswith these metals must be carried out in a reducing atmosphere ofhydrogen or other inert gas. At the conclusion of such tests, the heattreated powders can be scraped off the supports and investigated forchemical changes that might have occurred.

It is to be understood that various modifications and changes can bemade in the above exempliiication of this invention without departingfrom the spirit and scope thereof asset forth in the appended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. An apparatus for testing the thermal stability of specimenscomprising a metal support for the specimen, said support havinga-constricted portion serving as a high electrical resistance area, afixed electrically conductive terminal contact for securing one end ofsaid support, an electrically conductive, yieldingly movable headcontact for yieldingly securing the other end cf said support toaccommodate changes in the length of said support, a temperatureindicator adjacent said support for indieating the temperature of saidsupport, means for providing a non-luminous flame adjacent said support,a suction blower adjacent said support, a main electricalcircuit'adapted for connection to an electrical power source andincluding said terminal contact and said movable head contact in series,a voltmeter connected in parallel with said contacts, an ammeterconnected in series with said contacts, a switch in said circuit, anauxiliary circuit including a relay for operating said switch, a timerconnected in parallel with said relay, a switch in series with saidtimer, a relay in said main circuit in parallel with said contacts foroperating said timer switch.

2. An apparatus for testing thermal stability of a specimen, saidapparatus comprising a metal support adapted to have a specimen mountedthereon, an electrical circuit adapted for connection to a power sourceand including said support in series, electrical instruments in saidcircuit for indicating the electrical energy supplied to said support,an electrical timer in said circuit, means adjacent said support formaintaining a non-luminous flame, and a temperature indicator adjacentsaid support.

3. An apparatus for testing the thermal stability of a specimen, saidapparatus comprising a metal support adapted to have a specimen mountedthereon, an electrical circuit adapted for connection to a power sourceand including said support in series, igniting means spaced slightlyfrom said support, and a temperature indicator adjacent said support.

4. An apparatus for testing the thermal stability of a specimen, saidapparatus comprising a metal support adapted to have a specimen mountedthereon, an electrical circuit adapted lfor connection to a power sourceand including said support in series, means adjacent said support formaintaining a non-luminous flame, and a temperature indicator adjacentsaid support.

5. An electrical system for heating an object comprising a metalresistor adapted to support the object, a main electrical circuitadapted for connection to an electrical power source, said main circuitincluding said resistor in series, a voltmeter connected in parallelwith said resistor, an ammeter connected in series with said resistor, aswitch in said main circuit, an auxiliary circuit including a relay foroperating said switch, a timer connected in parallel with said relay, aswitch in series with said timer, and a relay in said main circuit inparallel with said resistor for operating said timer switch, said relaybeing connected in said main circuit between said resistor and said maincircuit switch.

6. An electrical system for heating an object comprising a metalresistor adapted to support the object, a main electrical circuitadapted for connection to an electrical power source, said main circuitincluding said resistor in series, electrical instruments in said maincircuit for indicating the electrical energy supplied to said resistor,a switch in said main circuit, an auxiliary circuit including a relayfor operating said switch,

'a timer connected .in :parallel with said relay, a

yswitch .in series with :said timer, and va relay in .said main Vcircuitin parallel With fsaid resistor 4for operating fsaid ltimer switch, saidrelay being connected in said main circuit Ybetween .said re- .sisterand said main circuit switch.

Y?. An apparatus :for testing .the thermal stability of a specimen,vsaid apparatus @comprising .an electrical resistor adapted lfor supporta specimen, an electrical circuit for energizing said re- :sistor tofea-use `heating of the resistor and specimen, and means adjacent r`said:support for maintaining -a non-luminous flame.

8. An apparatus .for testing the thermal sta-y bil-ity of ya specimen,said :apparatus comprising 12 anelectrical resistor adaptedio support a.speci-- men, an electrical circuit for energizing said resistor tocause heating of the resistor and specimen, and igniting means adjacentsaid resistor v:for ignitingfgases from the heated speci-men.

HOWARD R. MOORE.

vREFERENCES CITED The following references are .of record in `the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,291,409 Chubb et al Jan. 14,1919 1,789,098 De Graaf -Jan. 31, 1931

3. AN APPARATUS FOR TESTING THE THERMAL STABILITY OF A SPECIMEN, SAIDAPPARATUS COMPRISING A METAL SUPPORT ADAPTED TO HAVE A SPECIMEN MOUNTEDTHEREON, AN ELECTRICAL CIRCUIT ADAPTED FOR CONNECTION TO A POWER SOURCEAND INCLUDING SAID SUPPORT IN SERIES, IGNITING MEANS SPACED